1. Field of the Invention
The invention relates to an acceleration measuring apparatus that is used in automobiles, airplanes, industrial machines, cameras, portable terminals, medical equipment, watches, toys, game machines or the like for measuring vibrations, inclinations, travel distances and travel directions.
2. Description of the Related Art
Acceleration sensors have been widely used to measure vibrations, inclinations, travel distances and travel directions. Such acceleration sensors measure an electrical value, such as voltage, converted from a force produced due to an acceleration. However, since the sensitive section thereof has a variable characteristic because of production variation, the measured value cannot be used for any purpose as it is. Thus, the acceleration sensors have to be calibrated, by subjecting them to a known acceleration, so as to provide an output proportional to an acceleration.
The acceleration sensors have an x-axis directional sensor, a y-axis directional sensor and a z-axis directional sensor to measure respective accelerations in directions of three axes of orthogonal coordinates, that is, an x-axis, a y-axis and a z-axis. In calibration of sensitivity of such an acceleration sensor, each of the directional sensors has to be calibrated by sequentially aligning the x-, y- and z-axes with the gravitational acceleration direction. Japanese Patent No. 3,111,017 discloses a calibration method that reduces the inconvenience of such calibration involving calibrating each of the directional sensors by sequentially aligning the acceleration sensor with the three directions. According to the calibration method disclosed in the Japanese Patent, the acceleration sensor is mounted on a jig that allows components of a same magnitude of the gravitational acceleration to be applied to the x-axis, y-axis and z-axis directional sensors, so that sensitivities of the x-axis, y-axis and z-axis directional sensors can be calibrated simultaneously. According to this method, although the sensitivities, that is, outputs provided when an acceleration of 1G is applied to the sensor, can be calibrated simultaneously, a zero gravity state, that is, a zero-point cannot be calibrated.
In recent years, with the development of micromachine manufacture technology, highly sensitive acceleration sensors of the capacitance type and semiconductor piezo-resistor type that can detect an acceleration of 1G or lower have become popular. Such highly sensitive acceleration sensors are often used to detect not only vibrations but also inclinations, travel distances or travel directions, and thus, it is essential for such sensors to calibrate a zero-point output level.
Furthermore, since the acceleration sensors provide a weak output, the output is necessarily amplified by means of an amplifier. It is required to calibrate the sensitivity and zero point of the amplified output. In addition, an acceleration measuring apparatus incorporates a processing device that stores calibration parameters and calibration formulas and performs data processing using the parameters and calibration formulas. Thus, it is required to calibrate not only the output of the acceleration sensor but also the amplified output and the output of the processing device.
The invention, therefore, has an object to provide an acceleration measuring apparatus that is able to calibrate its output with a zero-point in the state that no acceleration is applied as well as sensitivity.
Another object of the invention is to provide an acceleration measuring apparatus comprising an acceleration sensor and a data processing device for dealing with the sensor output from the data processing device.
Further object of the invention is to provide an acceleration measuring apparatus that calculates the output with calibration including output variations by ambient temperature.
It would be apparent from the description of the invention below that the invention has further objects to provide a calibration method of the acceleration measured by the acceleration measuring apparatus.
An acceleration measuring apparatus according to the invention comprises an acceleration sensor that detects each component of an acceleration and produces an output based on each of the detected components in each axis direction of at least two mutually perpendicular axes of orthogonal coordinates for the acceleration sensor, a holding means that holds the acceleration sensor at at least two different positions, in which the acceleration sensor axes at one position each is at an angle, with the gravitational acceleration direction, different from that at the other position, and a processing circuit. The processing circuit develops calibration parameters based on the output by each component in the at least two axis directions of the gravitational acceleration measured by the acceleration sensor positioned at each of the at least two different positions and calibrates the output created by the acceleration sensor based on each of the detected components of the acceleration in each of the at least two axis directions by using the calibration parameters to provide a calibrated output.
It is preferable that the acceleration sensor detects each component of the acceleration in each direction of three mutually perpendicular axes of orthogonal coordinates for the acceleration sensor and creates the output based on each of the detected components, and that the holding means holds the acceleration sensor at two different positions in which each of the acceleration sensor axes at one position is at a angle, with the gravitational acceleration direction different, from that at the other position.
In the acceleration measuring apparatus as described above, it is desirable that the processing circuit further comprises a memory, stores the obtained calibration parameters in the memory, and calibrates the output created by the acceleration sensor based on each of the detected components of the acceleration in each of the three axis directions by using the stored calibration parameters to provide the calibrated output.
The processing circuit preferably calibrates the output created by the acceleration sensor based on each of the detected components of the acceleration in each of the three axis directions by using the sensitivity and the zero-point output in each of the three axis directions, according to the following equation:
Calibrated output=(the output based on each of the detected components of the accelerationxe2x88x92the zero-point output)/the sensitivity.
The acceleration measuring apparatus may further comprise a means for measuring an ambient temperature, and the processing circuit may develop temperature functions of calibration parameter, based on the output by each component in the three axis directions of the gravitational acceleration measured by the acceleration sensor positioned at each of the two different positions and calibrate the output created by the acceleration sensor based on each of the detected components of the acceleration by using the calibration parameters obtained with the ambient temperature by the temperature functions of calibration parameter to provide the calibrated output.
The temperature functions of calibration parameter preferably include a temperature function of output per unit magnitude of acceleration (hereinafter called xe2x80x9ctemperature function of sensitivityxe2x80x9d) in each of the three axis directions of the acceleration sensor and a temperature function of output of the acceleration sensor in each of the three axis directions without applied acceleration (hereinafter called xe2x80x9ctemperature function of zero-point outputxe2x80x9d).
In the acceleration measuring apparatus as described above, it is desirable that the processing circuit further comprises a memory, stores the obtained temperature functions of calibration parameter in the memory, and calibrates the output created by the acceleration sensor based on each of the detected components of the acceleration in each of the three axis directions by using calibration parameters at the ambient temperature obtained with the ambient temperature by the stored temperature functions of calibration parameter to provide the calibrated output.
The processing circuit preferably calibrates the output created by the acceleration sensor based on each of the detected components of the acceleration in each of the three axis directions by using the sensitivity and the zero-point output at the ambient temperature calculated with the ambient temperature by the temperature functions of sensitivity and the temperature functions of zero-point output, respectively, according to the following equation:
Calibration output=(the output based on each of the detected components of the accelerationxe2x88x92the zero-point output)/the sensitivity.